In conventional ultrasonic imaging, a focused beam of ultrasound energy is transmitted into body tissues to be examined and the returned echoes are detected and plotted to form an image. In echocardiography, the beam is usually stepped in increments of angle from a center probe position, and the echoes are plotted along lines representing the paths of the transmitted beams. In abdominal ultrasonography, the beam is usually stepped laterally, generating parallel beam paths, and the returned echoes are plotted along parallel lines representing these paths.
The basic principles of conventional ultrasonic imaging are described in the first chapter of Echocardiography, by Harvey Feigenbaum (Lippincott Williams & Wilkins, 5th ed., Philadelphia, 1993). It is well known that the average velocity ν of ultrasound in human tissue is about 1540 m/sec, the range in soft tissue being 1440 to 1670 m/sec (P. N. T. Wells, Biomedical Ultrasonics, Academic Press, London, New York, San Francisco, 1977). Therefore, the depth of an impedance discontinuity generating an echo can be estimated as the round-trip time for the echo multiplied by v/2, and the amplitude is plotted at that depth along a line representing the path of the beam. After this has been done for all echoes along all beam paths, an image is formed. The gaps between the scan lines are typically filled in by interpolation.
In order to insonify the body tissues, a beam formed by an array of transducer elements is scanned over the tissues to be examined. Traditionally, the same transducer array is used to detect the returning echoes. The use of the same transducer array to both produce the beam and detect returning echoes is one of the most significant limitations in the use of ultrasonic imaging for medical purposes; this limitation produces poor lateral resolution. Theoretically, the lateral resolution could be improved by increasing the aperture of the ultrasonic probe, but the practical problems involved with aperture size increase have kept apertures small and lateral resolution diminished. Unquestionably, ultrasonic imaging has been very useful even with this limitation, but it could be more effective with better resolution.
In the practice of cardiology, for example, the limitation on single aperture size is dictated by the space between the ribs (the intercostal spaces). For scanners intended for abdominal and other use, the limitation on aperture size is a serious limitation as well. The problem is that it is difficult to keep the elements of a large aperture array in phase because the speed of ultrasound transmission varies with the type of tissue between the probe and the area of interest. According to Wells (Biomedical Ultrasonics, as cited above), the transmission speed varies up to plus or minus 10% within the soft tissues. When the aperture is kept small, the intervening tissue is assumed to be homogeneous, and any variation is consequently ignored. When the size of the aperture is increased to improve the lateral resolution, the additional elements of a phased array may be out of phase and may actually degrade the image rather than improve it.
In the case of abdominal imaging, it has also been recognized that increasing the aperture size could improve the lateral resolution. Although avoiding the ribs is not a problem, beam forming using a sparsely filled array and, particularly, tissue speed variation needs to be compensated. With single aperture ultrasound probes, it has been commonly assumed that the beam paths used by the elements of the transducer array are close enough together to be considered similar in tissue density profile, and therefore that no compensation was necessary. The use of this assumption, however, severely limits the size of the aperture that can be used.
The problems of limited total aperture size have been addressed by the development of multiple aperture ultrasound imaging techniques as shown and described for example in U.S. Pat. No. 8,007,439, and US Patent Application Publication 2011/0201933.